"This spinal structure would have enabled pregnant women to balance their weight over the hips," said study leader David Lewis, a UT Austin alumnus and now a psychologist at Bilkent University in Turkey.

"These women would have been more effective at foraging during pregnancy and less likely to suffer spinal injuries," he said in a UT Austin news release. "In turn, men who preferred these women would have had mates who were better able to provide for foetus and offspring, and who would have been able to carry out multiple pregnancies without injury."

Evolved standard of attractiveness

This preference evolved over thousands of years and is likely to persist for a long time, the researchers noted.

Their conclusions came from a two-part study in which 100 men were asked to look at images of women and rate their attractiveness. Women with a 45.5-degree in the lower back were rated as most appealing. The researchers next determined from a group of 200 men that guys preferred this degree of lower back curvature regardless of a woman's butt size.

"What's fascinating about this research is that it is yet another scientific illustration of a close fit between a sex-differentiated feature of human morphology – in this case lumbar curvature – and an evolved standard of attractiveness," study co-author David Buss, a UT Austin psychology professor, said in the news release.

"This adds to a growing body of evidence that beauty is not entirely arbitrary, or 'in the eyes of the beholder' as many in mainstream social science believed, but rather has a coherent adaptive logic," he said.

The study was published online in the journal Evolution and Human Behaviour.

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